Summary: Invariant natural killer T (iNKT) cells have been shown to promote resistance to a variety of viral infections. Despite their cytopathic-sounding name, it does not seem that the anti-viral effects of iNKT cells are due to their killing of infected cells. Instead, iNKT cells function as cellular adjuvants that promote anti-viral responses by other lymphocytes, including antigen-specific T cells. iNKT cells are an innate T cell population that is present in all individuals, and that utilizes a conserved TCR that recognizes lipid patterns presented by non-polymorphic CD1d molecules. As a result of these features, iNKT cells can be targeted in genetically diverse human populations using a single therapeutic strategy (e.g. anti-TCR antibodies, synthetic lipid antigens). Thus, iNKT cells could be exploited as a generic (i.e. HLA-independent) strategy to promote anti- viral antigen-specific T cell responses. The goal of this project is to provide mechanistic data that will support the development of human iNKT cells as broad anti-viral agents. The mechanisms involved in human iNKT- mediated adjuvancy in vivo will be investigated using a model of Epstein-Barr virus infection, in which autologous T cells control the degree of virally-driven B cell hyperplasia. Our preliminary studies show that administering iNKT cells at late time points after viral infection is associated with clearance of B- lymphoproliferative masses and with enhanced antigen-specific T cell responses. We will investigate two specific hypotheses about the mechanisms by which iNKT cells mediate these effects: i) by conditioning of monocytic APCs in a way that diminishes their immunosuppressive properties and/or enhances their immunostimulatory features; ii) by directly activating T cells in a way that enables them to overcome suppressive signals. Aim1 will determine iNKT cell activation requirements; Aim 2 will ascertain the importance of iNKT cell interactions with monocytic APCs; Aim 3 will assess the impact of iNKT cells on T cells. These studies will significantly advance our understanding of the cellular and molecular pathways involved in iNKT-mediated adjuvancy, and will thus guide the development of clinical strategies to engage iNKT cells to promote anti-viral immunity.